Interpretive Summary: Long-term agricultural sustainability is severely threatened by widespread use of classical insecticides. Threats include increasing resistance to insecticides and sharply decreasing environmental quality. The concept of biological control of insects is a potentially powerful alternative to classical insecticides. Biological control is based on the idea that direct application of certain insect-specific predators, pathogens and parasites can reduce pest insect populations to a point that the pests exert only negligible economic damage. A major problem, however, is that many insect parasites have a very narrow range of hosts. This limits the usefulness of any given parasite species in biological control programs. In this paper, we examined the relationship of a parasitic insect and its host, a pest insect. We learned that venom from the parasite is essential to the success of the parasite. Scientists who study host/parasite relationships will use this information in future research designed to understand how parasite venom can be manipulated to broaden the host range of a parasite. Ultimately, this research will benefit growers who produce vegetable crops and the people who consume vegetables.

Technical Abstract:
The relationships between parasitoids and their insect hosts have attracted attention at two levels. First, the basic biology of host-parasitoid interactions is of fundamental interest. Second, parasitoids have tremendous potential as biological control agents in sustainable agriculture programs. Pteromalus puparum differs from many parasitoids because females inject venom, but no other factors, while depositing eggs into hosts. In this study, we tested our hypothesis that P. puparum venom influences host gene expression in the two main immunity-conferring tissues in insects, hemocytes and fat body. At 1 h post venom injection we recorded significant decreases in transcript levels of 217 EST clones (revealing 113 putative genes, including 62 unknown contigs) derived from forward subtractive libraries of host hemocytes and in transcript levels of 288 EST clones (221 putative genes, including 123 unknown contigs) from libraries of host fat body. These genes are related to insect immune response, cytoskeleton, cell cycle and apoptosis, respiration and energy metabolism, material metabolism and transport, stress response and transcriptional and translational regulation. We verified the reliability of the SSH data with semi-quantitative RT-PCR analysis of a set of randomly selected genes. This analysis showed that all the selected genes were down-regulated after venom injection. Our findings strongly support our hypothesis that P. puparum venom influences gene expression in host hemocytes and fat body.